This invention relates to a process for the production of an adherent metal oxide coating on a solid body comprised of a chemically inert, porous material, such as a ceramic monolithic catalyst carrier, whereby metal oxide in a pulverized form is dispersed in a liquid medium, the body is dipped into this dispersion and dried.
It is known to produce dispersions from particles having very small grain diameters (&lt;&lt; 0.1 .mu.m). If, however, one wants to disperse particles with a larger grain diameter (&gt; 1 .mu.m) in a liquid medium in order, for example, to apply a metal oxide powder in a thin layer on a ceramic monolith to be used for the purification of exhaust gases from an internal combustion engine, then considerable difficulties arise because of decreased stability of the dispersion. That is, the metal oxide powder settles after a short time. As a result, the dispersion no longer has the initial characteristics necessary for an adequate distribution of the metal oxide on the body; for example, there is not a uniform and sufficient degree of dispersion of metal oxide particles in the liquid medium. Continuous processes, such as those employed in the industrial production of catalysts, cannot be carried out with such unstable dispersions. While the dispersion can be kept in continuous motion in order to avoid these disadvantages, this requires an increased expenditure in apparatus.
Furthermore, many dispersions of solid substances in a liquid medium show only a limited stability range when interfering ions are present. If this range is too narrow, then a destabilization of the dispersion can occur very quickly when impurities are present.
A further disadvantage of the known dispersions is the low adhesive strength of the metal oxide powder applied to a body.
Accordingly, there exists a need in the art for a dispersion which can be processed easily, which has a high stability in the presence of interfering ions, and which imparts excellent adhesive strength to the metal oxide layer formed on a solid body.